Re: [Sbcl-commits] master: Recognize (integer-length (ldb (byte 64 0) (1- (logand n (- n))))) as ctz

Here's what it now says

The bit index is an unsigned offset from bit 0 of the source operand.
If the content of the source operand is zero, the destination operand
is unmodified.1

1: With a zero source operand on some older processors, use of a
32-bit operand size may clear the upper 32 bits of a 64-bit
destination while leaving the lower 32 bits unmodified.

And AMD says

If the second operand contains 0, the instruction sets ZF to 1 and
does not change the contents of the destination register

On Fri, Jan 9, 2026 at 8:29 PM Stas Boukarev <[email protected]> wrote:
>
> If you look at the new intel documentation they've changed that it
> does indeed not touch the original result, even going back to old cpus
> (unless maybe something from i386), except some not quite old cpus do
> touch 32-bit registers.
> clang does that as well.
>
> On Fri, Jan 9, 2026 at 8:22 PM Douglas Katzman via Sbcl-devel
> <[email protected]> wrote:
> >
> > I see what you're trying to do to support older CPUs that don't implement TZCNT, however this MOV is not technically correct.
> > If the processor treats TZCNT just like BSF, then the destination register is undefined (https://www.felixcloutier.com/x86/bsf), not unaffected when the input is 0.  So the output could be zero or random bits.  I realize that clang uses this technique, but my guess is they simply haven't gotten complaints. gcc takes the manufacturer's spec at face value, and uses a pedantically correct solution to fixup the result if the input was zero
> >  testq %rdi, %rdi
> >  movl $64, %eax
> >  cmovne %edx, %eax
> >
> >>
> >>
> >> +(define-vop ()
> >> +  (:translate count-trailing-zeros)
> >> +  (:policy :fast-safe)
> >> +  (:args (arg :scs (signed-reg unsigned-reg) :to :save))
> >> +  (:arg-types (:or signed-num unsigned-num))
> >> +  (:results (res :scs (unsigned-reg)))
> >> +  (:result-types unsigned-num)
> >> +  (:generator 5
> >> +    (inst mov res 64)
> >> +    (inst tzcnt res arg)))
> >
> > _______________________________________________
> > Sbcl-devel mailing list
> > [email protected]
> > https://lists.sourceforge.net/lists/listinfo/sbcl-devel


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